Abstract
Much evidence indicates that the pineal gland and thesuprachiasmatic nucleus (SCN) are the primary pacemakers in the housesparrow, Passer domesticus. The interactions between the pineal andSCN predicted by the neuroendocrine loop model indicates that uncouplingwould cause the two oscillators to damp out in constant darkness. Basedupon the original neuroendocrine loop model, a mathematical frameworkof the house sparrow circadian regulatory organization that incorporatesdamping and co-inhibitory coupling has been formulated. The proposedmodel clearly indicates that two coupled oscillators must be 180(°)out of the phase for sustaining oscillations. From damping coefficients,which can be determined from experimental data, other parameters suchas external stimuli (interaction coefficient) and characteristicfrequencies can then be computed. Based upon earlier studies and simulations,we conclude that the sparrow pineal gland dampens more rapidly than does theSCN, suggesting that the SCN are probably more important in sparrowsthan previously thought. The model also provides the explanations ofendogenous circadian period (tau) alteration. Finally, we extend this modelto other avian and to mammalian circadian systems. We suggest that avianand mammalian circadian systems may differ in damping coefficients ofpineal glands and the degree of SCN dominance.